On Wed, 2014-01-22 at 13:39 -0500, Ric Wheeler wrote: > On 01/22/2014 01:35 PM, James Bottomley wrote: > > On Wed, 2014-01-22 at 13:17 -0500, Ric Wheeler wrote: [...] > >> I think that the key to having the file system work with larger > >> sectors is to > >> create them properly aligned and use the actual, native sector size as > >> their FS > >> block size. Which is pretty much back the original challenge. > > Only if you think laying out stuff requires block size changes. If a 4k > > block filesystem's allocation algorithm tried to allocate on a 16k > > boundary for instance, that gets us a lot of the performance without > > needing a lot of alteration. > > The key here is that we cannot assume that writes happen only during > allocation/append mode. But that doesn't matter at all, does it? If the file is sector aligned, then the write is aligned. If the write is short on a large block fs, well we'd just have to do the RMW in the OS anyway ... is that any better than doing it in the device? > Unless the block size enforces it, we will have non-aligned, small > block IO done > to allocated regions that won't get coalesced. We always get that if it's the use pattern ... the question merely becomes who bears the burden of RMW. > > It's not even obvious that an ignorant 4k layout is going to be so > > bad ... the RMW occurs only at the ends of the transfers, not in the > > middle. If we say 16k physical block and average 128k transfers, > > probabalistically we misalign on 6 out of 31 sectors (or 19% of the > > time). We can make that better by increasing the transfer size (it > > comes down to 10% for 256k transfers. > > This really depends on the nature of the device. Some devices could > produce very > erratic performance Yes, we get that today with misaligned writes to the 4k devices. > or even (not today, but some day) reject the IO. I really doubt this. All 4k drives today do RMW ... I don't see that changing any time soon. > >> Teaching each and every file system to be aligned at the storage > >> granularity/minimum IO size when that is larger than the physical > >> sector size is > >> harder I think. > > But you're making assumptions about needing larger block sizes. I'm > > asking what can we do with what we currently have? Increasing the > > transfer size is a way of mitigating the problem with no FS support > > whatever. Adding alignment to the FS layout algorithm is another. When > > you've done both of those, I think you're already at the 99% aligned > > case, which is "do we need to bother any more" territory for me. > > > > I would say no, we will eventually need larger file system block sizes. > > Tuning and getting 95% (98%?) of the way there with alignment and IO > scheduler > does help a lot. That is what we do today and it is important when > looking for > high performance. > > However, this is more of a short term work around for a lack of a > fundamental > ability to do the right sized file system block for a specific class > of device. > As such, not a crisis that must be solved today, but rather something > that I > think is definitely worth looking at so we can figure this out over > the next > year or so. But this, I think, is the fundamental point for debate. If we can pull alignment and other tricks to solve 99% of the problem is there a need for radical VM surgery? Is there anything coming down the pipe in the future that may move the devices ahead of the tricks? James -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
